In optical communications networks, electro-optical assemblies are used to transmit and receive optical signals over optical fibers. A typical electro-optical assembly (EA) comprises a transmitter optical subassembly (TOSA), a receiver optical subassembly (ROSA) and an electronic subassembly (ESA). The TOSA and ROSA normally are implemented using traditional transistor outline (TO)-can or fiber optic transceiver (FOT) lead frame architectures. The TOSA and ROSA TO-cans typically each comprise optics that are mounted on top of the TO-can. On the transmitter side, the TOSA TO-can includes a light source that is mounted below the TOSA optics and in optical alignment with the optics. The light source, which is typically a laser diode or light emitting diode (LED), generates modulated optical signals that carry data, which are then coupled by the TOSA optics into an end of an optical fiber for transmission over an optical fiber network. The light source is wire bonded to pins of the TOSA TO-can. The pins of the TOSA TO-can are then soldered to a printed circuit board (PCB) of the ESA. The TOSA FOT includes a light source that is mounted and wire bonded directly on a lead frame, which is encapsulated in a clear mold with a lens. The leads of the FOT lead frame are then soldered to ESA.
The ESA typically also includes a controller IC, a transmitter driver IC, a receiver IC and passive components, such as, for example resistors, capacitors and inductors, all of which are electrically connected to conductors of the PCB of the ESA. The controller IC delivers electrical signals to the transmitter driver IC for controlling the modulation and bias currents of the light source in the TOSA TO-can or FOT.
On the receiver side, a photodiode of the ROSA TO-can is mounted below the ROSA optics in optical alignment with the ROSA optics. The ROSA optics receive an incoming optical signal output from the end of a receive optical fiber and direct the light output from the end of the receive optical fiber onto the active area of the photodiode. The photodiode is wire bonded to pins of the ROSA TO-can. The pins of the ROSA TO-can are soldered to the PCB of the ESA. As for the ROSA FOT, the photodiode is mounted directly on to the lead frame of the ROSA FOT. The leads of ROSA FOT leadframe are then soldered to the PCB of the ESA. During operation, the photodiode converts the incoming optical signal into an electrical signal, which is then processed by the receiver IC and the controller IC of the ESA.
The TO-can and FOT lead frame architectures described above are relatively large and relatively difficult to mechanically package. The signal paths tend to be relatively long due to the relatively long pins of the TO-cans and long leads of the FOTs, which can result in the EA having impedance matching issues, interference issues by external electromagnetic (EM) signals on the receiver side, and excessive EM emission issues on the transmitter side. Furthermore, the encapsulation of a FOT might crack or delaminate due to a large coefficient of thermal expansion (CTE) mismatch between the metal lead frame and the mold material.
It would be desirable to provide an EA in which the electrical circuits and other components of the ROSA, the TOSA and the optics are integrated together on a single PCB and encapsulated in a single molded EA package. Integrating the ROSA and the TOSA on a single PCB would reduce reliability issues associated with wire bonding and would allow signal path lengths and the overall size of the EA to be reduced. Furthermore, it would be desirable to provide an EA that is encapsulated in an encapsulation material having a CTE that is relatively close to the CTE of the PCB such that cracking and delamination due to temperature changes are avoided.